As the economic impact of the coronavirus outbreak ripples out from the epicenter in China, airlines have been among the hardest hit.

Already, the outbreak is leading to sharp reductions in air travel demand in the US, with major companies such as Amazon ordering a freeze on all non-essential employee travel. The result is clear in photos showing empty airplane seats and deserted terminals.

In China, the impact has been even more severe, with air travel plummeting 80 percent at the country's busiest airports and mass cancellations of both domestic and international flights.

The reduction in global airline capacity, measured by how many seats remain grounded, is now greater than after the terrorist attacks of September 11, 2001, industry analysts say.

After 9/11, airline revenue dropped an estimated $19.6 billion in 2002 dollars. The coronavirus crisis could cost the industry an estimated $29.3 billion in lost revenue for 2020, according to the industry group IATA.

The group said on Friday that countries with confirmed virus cases in excess of 90 —China, Italy, Iran, Japan, Singapore and South Korea - represent 25 percent of global airline passenger numbers and 20 percent of global passenger revenues.

'Back after 9/11 at the end of 2001, it really took about nine months before we saw the industry recover from the impact of the events,' said Wall Street Journal aviation correspondent Benjamin Katz.

'Now, with the coronavirus, it's a very different situation and it's difficult to give an assessment, but analysts are expecting with the coronavirus this could actually last quite a bit longer,' he continued.

With the extent of the crisis still unknown, experts cautioned that the full impact on airlines remains to be seen.

'If we look back in ten years time, will this be seen as a blip or a game changer?' IBA Aviation Consultancy CEO Phil Seymour told the Journal.

On Friday, the flight cancellations continued around the globe, with United Airlines sharply cutting flights to Japan and South Korea.

United ended down 5.2 percent on Friday and were down more than 22 percent over the last week.

The Chicago-based airline said near-term demand to China has almost disappeared and demand to the rest of its trans-Pacific routes has dropped by 75 percent, United said on Monday.

Among U.S. airlines, United has the biggest international exposure, drawing about 40 percent of its revenues from overseas flights.

Earlier this week, Delta cut also South Korea flights in half, citing the outbreak and plummeting demand.

Even domestically in the U.S., airlines are starting to see slumping demand, as companies reconsider the need for business travel and conferences on the cusp of a potential outbreak.

On Friday, Amazon deferred all non-essential travel, within the U.S. and beyond, and Google set new restrictions for travel to South Korea and other places, as corporations moved to protect employees from the spread of coronavirus.

Coronavirus fears have intensified in recent days since countries besides China have reported a sharp increase in cases, with six countries reporting their first cases.

Amazon is one of the latest companies to clamp down on travel because of the outbreak, which has caused at least 2,797 deaths globally.

Google is banning travel to Iran and two Italian regions, Lombardy and the Veneto, where the virus is spreading. The company will also ban travel to South Korea and Japan, from March 2, a spokesman for the company confirmed.

Canada's TD Bank Group told Reuters it was suspending all non-essential business travel to China, Iran, Italy, Japan, South Korea and Singapore. Its peer Bank of Nova Scotia has also reportedly halted non-essential travel.

JetBlue which flies in the United States, the Caribbean and Latin America, became the first U.S. airline to offer waivers for all travel on Wednesday, announcing it would suspend change and cancellation fees for new flight bookings between February 27 and March 11 this year.

Facebook also said it would cancel its annual developer conference in May because of the virus.

Among those who are still traveling, there are concerns about the screening process for international travelers at John F. Kennedy Airport in New York.

Students Emily Ferrara and Blair Haworth told WCBS-TV they had just returned from Florence, Italy, where their study abroad program was canceled because of the virus outbreak, which has infected at least 300 in that country.

Yet, the students said they weren't asked a single question about potential coronavirus symptoms once they landed in New York City.

'We didn't even get checked. Like, we're used to being in Florence where you get your temperature checked. Here they didn't do anything, which is kind of crazy,' Ferrara said.

'Considering, like, how much the cases have spread so fast, like, they should definitely be taking more precautions here.'